1. Field of the Invention
The present invention relates to a method of producing a dielectric ceramic composition used, for example, as a dielectric layer of a multi-layer ceramic capacitor and a method of producing an electronic device using the dielectric ceramic composition as a dielectric layer.
2. Description of the Related Art
A multi-layer ceramic capacitor as an example of electronic devices is formed by printing a conductive paste on a green sheet comprised of a predetermined dielectric ceramic composition, layering a plurality of the green sheets printed the conductive paste thereon and firing the green sheet and internal electrodes as one body.
A dielectric ceramic composition had a characteristic that it was reduced and became semiconductive when fired in a neutral or reducing atmosphere having a low oxygen partial pressure. Therefore, when producing a multi-layer ceramic capacitor, it was inevitable to fire in an oxidizing atmosphere having a high oxygen partial pressure. Accordingly, as an internal electrode material to be fired at a time with the dielectric ceramic composition, it is necessary to use an expensive noble metal, such as palladium and platinum, which does not melt at a temperature of sintering the dielectric ceramic composition and is not oxidized when being fired in an oxidizing atmosphere, so it has been a large obstacle for lowering the cost of the multi-layer ceramic capacitor to be produced.
On the other hand, to use an inexpensive base metal, such as nickel and copper, as an internal electrode material, it is necessary to develop a dielectric ceramic composition having characteristics of not becoming semiconductive by being fired at a low temperature in a neutral or reducing atmosphere, that is having excellent resistance to reducing, showing sufficient specific dielectric constant and excellent dielectric characteristics (for example, having a small capacity-temperature change rate) after firing.
A variety of proposals have been made for dielectric ceramic composition wherein a base metal can be used as an internal electrode material in the related art.
For example, The Japanese Unexamined Patent Publication No. 1988-224108 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by (Sr1-xCax)m(Ti1-yZry)O3 (note that 0.30xe2x89xa6xxe2x89xa60.50, 0.03xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and containing as subcomponents 0.01 to 2.00 parts by weight of Mn (converted to MnO2) and 0.10 to 4.00 parts by weight of SiO2 with respect to 100 parts by weight of the main component.
The Japanese Unexamined Patent Publication No. 1988-224109 discloses a dielectric ceramic composition further containing ZnO in an amount of 0.01 to 1.00 parts by weight with respect to the above main component in addition to the above Mn and SiO2.
The Japanese Unexamined Patent Publication No. 1992-206109 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by (Sr1-xCax)m(Ti1-yZry)O3 (note that 0.30xe2x89xa6xxe2x89xa60.50, 0.00xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and the particle diameter is made to be in the range of 0.1 to 1.0 xcexcm.
The Japanese Examined Patent Publication No. 1987-24388 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by MeOk TiO2 (note that Me is a metal selected from Sr, Ca and Sr+Ca, and k is 1.00 to 1.04) and 0.2 to 10.0 parts by weight of a glass component with respect to 100 parts by weight of the main component wherein Li2O, M (note that M is at least one kind of metal oxide selected from BaO, CaO and SrO) and SiO2 are used at a predetermined mole ratio.
The Japanese Unexamined Patent Publication No. 1984-227769 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by {(Sr1-xCax)O}kxe2x80xa2TiO2 (note that 0xe2x89xa6xxe2x89xa61.00, 1.00xe2x89xa6kxe2x89xa61.04) and 0.2 to 10.0 parts by weight of a glass component with respect to 100 parts by weight of the main component wherein Li2O, M (note that M is at least one kind of metal oxide selected from BaO, Cao and SrO) and SiO2 are used at a predetermined mole ratio.
The Japanese Unexamined Patent Publication No. 1988-224106 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by (Sr1-xCax)m (Ti1-yZry)O3 (note that 0.30xe2x89xa6xxe2x89xa60.50, 0.03xe2x89xa6yxe2x89xa60.20, 0.95xe2x89xa6mxe2x89xa61.08) and containing as a subcomponent 0.01 to 2.00 parts by weight of Mn (converted to MnO2), 0.10 to 4.00 parts by weight of SiO2 and 0.01 to 1.00 parts by weight of Mo (converted to MoO) with respect to 100 parts by weight of the main component.
The Japanese Unexamined Patent Publication No. 1992-14704 discloses a dielectric ceramic composition containing as a main component a dielectric oxide having a composition expressed by (SrxCa1-x)(ZryTi1-y)O3 (note that 0.59xe2x89xa6xxe2x89xa60.65, 0xe2x89xa6yxe2x89xa60.1) and containing less than 3.0 parts by weight of SiO2 (note that 0 part by weight is not included) with respect to 100 parts by weight of the main component.
There have been disadvantages, however, that when producing the dielectric ceramic composition described in the publications in a usual method, an accelerating lifetime of insulation resistance after firing is insufficient in all cases, while, when producing a multi-layer ceramic capacitor having an internal electrode made by a base metal, such as nickel, by using the dielectric ceramic composition, the multi-layer ceramic capacitor to be obtained has lower credibility.
Note that the present inventors have proposed to add a rare-earth component, such as yttrium, in a composition system expressed by {(Sr1-xCax)O}m. (Ti1-yZry)O2 wherein the mole ratio m is relatively low so that the accelerating lifetime of insulation resistance is improved (The Japanese Patent Application No. 2000-187800). However, when producing in a usual method, it was difficult to attain a longer lifetime even by adding a rare-earth component in a range where the mole ratio m satisfies 0.995xe2x89xa6m less than 1.08 which is relatively high.
An object of the present invention is to provide a method of producing a dielectric ceramic composition having excellent resistance to reducing during firing and excellent capacity-temperature characteristics after firing, which furthermore improves an accelerating lifetime of insulation resistance and a method of producing an electronic device, such as a chip capacitor, having improved credibility.
To attain the above object, according to the present invention, there is provided a method of producing a dielectric ceramic composition containing a main component expressed by a composition formula of {(Sr1-xCax)O}m. (Ti1-yZry)O2 wherein the mole ratio m satisfies 0.94 less than m less than 1.08, the code x satisfies 0xe2x89xa6xxe2x89xa61.00 and the code y satisfies 0xe2x89xa6yxe2x89xa60.20; and
a fourth subcomponent including an oxide of R (note that R is at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu);
characterized by producing the dielectric ceramic composition by using a composition source material wherein at least a part of a fourth subcomponent source material for the fourth subcomponent is brought to react in advance with a main component source material for the main component.
According to the present invention, there is provided a method of producing an electronic device comprising a dielectric layer constituted by a dielectric ceramic composition containing a main component expressed by a composition formula of {(Sr1-xCax)O}m. (Ti1-yZry)O2, wherein the mole ratio m satisfies 0.94 less than m less than 1.08, the code x satisfies 0xe2x89xa6xxe2x89xa61.00 and the code y satisfies 0xe2x89xa6yxe2x89xa60.20; and a fourth subcomponent including an oxide of R (note that R is at least one selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu);
characterized by producing the dielectric ceramic composition by using a composition source material wherein at least a part of a fourth subcomponent source material for the fourth subcomponent is brought to react in advance with a main component source material for the main component.
It is considered that a distribution of the fourth subcomponent becomes more uniform (solute state) after firing by bringing at least a part of the fourth subcomponent source material react with the main component source material in advance.
Preferably, the oxide of R included in the fourth subcomponent material is at least one oxide of Sc, Y, Ce, Dy, Ho, Er, Tm, Yb and Lu.
A method of the reaction is not limited to a solid phase method, such as a calcining method, but a liquid phase method, such as an oxalate method, hydrothermal synthesis method, sol-gel method, etc. may be used. Note that the description xe2x80x9cat least a partxe2x80x9d means at least a part of the fourth subcomponent source material corresponding to the whole amount of the fourth subcomponent to be contained in the final composition. Note that the whole amount of the fourth subcomponent to be contained in the final composition is preferably reacted.
Preferably, the fourth subcomponent source material is brought to react in advance in an amount of 0.02 mole or more and less than 2 moles (calculated as a conversion mole value of the R in the oxide) with respect to 100 moles of the main component source material.
Preferably, the main component source material is expressed by a composition formula of {(Sr1-xCax)O}mxe2x80x2. (Ti1-yZry)O2 and the mole ratio mxe2x80x2 in said composition formula with respect to the mole ratio m of the final composition satisfies mxe2x80x2xe2x89xa6m.
Preferably, the dielectric ceramic composition is produced by firing after adding to said composition source material a substance containing at least one element of Sr and Ca.
More preferably, the dielectric ceramic composition is produced by firing after adding to said composition source material a substance containing at least one element of Sr and Ca without adding a substance containing Ti.
Preferably, the mole ratio mxe2x80x2 of the composition formula in the main component source material satisfies 0.9 less than mxe2x80x2.
It has been proven by the present inventors that it is difficult to improve an accelerating lifetime (highly accelerated life time) of insulation resistance when producing by a usual method a dielectric ceramic composition containing a main component expressed by a composition formula of {(Sr1-xCax)O}m. (Ti1-yZry)O2 wherein a mole ratio m satisfies (0.995xe2x89xa6m less than 1.08) and a particular fourth subcomponent. The reason thereof is considered that R or an oxide of R in the fourth subcomponent source material segregates much at a grain boundary portion and at a triple point in the dielectric ceramic composition after firing and is not uniformly distributed in the grains.
In the method of producing a dielectric ceramic composition according to the present invention, by producing the above dielectric ceramic composition by using a composition source material wherein at least a part of the fourth subcomponent source material is brought to react with a main component source material in advance, it is possible to produce a dielectric ceramic composition having resistance to reducing during firing, excellent capacity-temperature characteristics after firing, and furthermore, an accelerating lifetime of insulation resistance (for example, 200xc2x0 C. and DC8V/xcexcm) due to a uniform distribution of R or an oxide of R of the fourth component material in the grain boundary in the dielectric ceramic composition after firing.
Particularly, when using a main component source material expressed by a composition formula of {(Sr1-xCax)O}mxe2x80x2. (Ti1-yZry)O2 having a smaller mole ratio mxe2x80x2 than the mole ratio m after firing, R or an oxide of R of the fourth subcomponent source material is more easily distributed uniformly in the dielectric ceramic composition after firing. Consequently, the accelerating lifetime of insulation resistance of the dielectric ceramic composition to be obtained improves more.
Note that it is proven by the present inventors that in the range where the mole ratio m in the above composition formula is relatively low (0.94 less than m less than 0.995), the fourth subcomponent is almost uniformly distributed in the main component in the dielectric ceramic composition after firing even the fourth subcomponent source material is not brought to react with the main component source material. However, even in such a region where the mole ratio m is relatively low, it becomes easier for the fourth subcomponent source material to uniformly distributed in the grains of the main component by producing the above dielectric ceramic composition by using a component material wherein the fourth subcomponent source material is brought to react with the main component source material comparing with a case of using a material wherein the fourth subcomponent source material is not brought to react with the main component source material in advance, that is, the fourth subcomponent source material is subsequently added to the main component source material. As a result, an improvement of the accelerating lifetime of insulation resistance can be expected.
In a production method according to the present invention, an electronic device, such as a chip capacitor, having excellent capacity-temperature characteristics, an improved accelerating lifetime of insulation resistance and an improved reliability can be produced.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-93415 (filed on March 30) and No. 2000-204723 (filed on July 6), the disclosure of which is expressly incorporated herein by reference in its entirety.